The present invention relates to a hydrolyzed ethylene-vinyl acetate copolymer having an improved water resistance, and more particularly to a hydrolyzed ethylene-vinyl acetate copolymer having an excellent gas impermeability even under high humidity.
A hydrolyzed ethylene-vinyl acetate copolymer is excellent in an oxygen impermeability, a mechanical strength, and the like, and is useful for various uses such as films, sheets, containers and fibers. However, the hydrolyzed copolymer has defects that physical properties such as a flexibility of the molded articles therefrom are changed largely due to change of humidity and temperature of the outside, and a gas impermeability is particularly changed largely due to the change of temperature and it is lowered under high humidity, since the hydrolyzed copolymer has a high moisture absorption.
In order to solve the above defects, there have been proposed, for instance, (1) a method in which a film or sheet of a hydrolyzed ethylene-vinyl acetate copolymer is laminated with a water resistant resin such as a polyolefin to prevent a permeation of water-vapor from the outside, and (2) a method in which a crosslinkable group such as silyl group are introduced into a structure of the hydrolyzed ethylene-vinyl acetate copolymer to provide water resistance for the hydrolyzed copolymer, and the like.
However, in the laminating method (1), a laminating operation is required and even if laminating the film or sheet of the hydrolyzed copolymer with the water resistant resin, it is difficult to prevent the permeation of water-vapor from the edges of the film or sheet. On the other hand, in the method (2), the crosslinking or gelation of the hydrolyzed copolymer occurs easily upon the preparation or molding of the hydrolyzed copolymer, and the moldability is lowered.
An object of the present invention is to provide a hydrolyzed ethylene-vinyl acetate copolymer having an improved water resistance without the abovementioned defects.
This and other objects of the present invention will become apparent from the description hereafter.